Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

Provided is a light-emitting element having a light-emitting layer which contains at least a host material and a plurality of guest materials, where the host material has a lower T1 level than that of at least one of the plurality of guest materials. The emission of the one of the plurality of guest materials exhibits a multicomponent decay curve, and the lifetime thereof is less than or equal to 15 μsec, preferably less than or equal to 10 μsec, more preferably less than or equal to 5 μsec, where the lifetime is defined as a time for the emission to decrease in intensity to 1/100 of its initial intensity.

TECHNICAL FIELD

One embodiment of the present invention relates to a light-emittingelement in which an organic compound that emits light by application ofan electric field is provided between a pair of electrodes, and alsorelates to a light-emitting device, an electronic device, and a lightingdevice including such a light-emitting element.

BACKGROUND ART

A light-emitting element using an organic compound as a luminous body,which has features such as thinness, lightness, high-speed response, andDC drive at low voltage, is expected to be applied to a next-generationflat panel display. In particular, a display device in whichlight-emitting elements are arranged in a matrix is considered to haveadvantages in a wide viewing angle and excellent visibility over aconventional liquid crystal display device.

A light-emitting element is considered to have the following emissionmechanism: when voltage is applied between a pair of electrodes with anEL layer containing a light-emitting substance provided therebetween,electrons injected from a cathode and holes injected from an anode forman excited state in an emission region of the EL layer, and energy isreleased and light is emitted when the excited state returns to a groundstate. In the case of using an organic compound as a light-emittingsubstance, there can exist in two types of excited states: a singletexcited state and a triplet excited state. Luminescence from the singletexcited state (S1) is referred to as fluorescence, and luminescence fromthe triplet excited state (T1) is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered that S1:T1=1:3.

Development for improving element characteristics has been conducted;for example, a light-emitting element having a structure utilizing notonly fluorescence but also phosphorescence has been developed. In alight-emitting layer of the light-emitting element, a host material anda guest material are contained, and a phosphorescent material exhibitinghigh energy emission is used as the guest material (e.g., see PatentDocument 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2010-182699

DISCLOSURE OF INVENTION

In general, it is thought that to improve the emission efficiency of alight-emitting element using a host material and a guest material, theT1 level of the host material is preferably higher than that of theguest material. In the case where a phosphorescent compound having highemission energy (e.g., a blue phosphorescent compound) is used as aguest material, a host material needs to have higher T1 level than inthe case where phosphorescent compound having lower emission energy isused as a guest material; thus, its reliability as a host materialdecreases.

In view of the above background, one embodiment of the present inventionprovides a novel light-emitting element which shows high reliabilityeven if a phosphorescent compound having high emission energy isemployed. Specifically, disclosed is a light-emitting element includinga light-emitting layer containing at least a host material and aplurality of guest materials, where emission obtained from thelight-emitting layer (including photoluminescence (PL) byphotoexcitation or electroluminescence (EL) by electric fieldexcitation) exhibits a multicomponent decay curve expressed by Formula I(provided that i≠1) and the emission lifetime is short enough to prevailthe thermal deactivation process of the host material. Note that, in thespecification and claims, the emission lifetime means a time requiredfor the emission intensity to decrease to 1/100 of the initial value.The emission lifetime which is short enough to prevail the thermaldeactivation process of the host material is specifically more than orequal to 5 μsec and less than or equal to 15 μsec, preferably more thanor equal to 5 μsec and less than or equal to 10 μsec. The excitonconcentration in the light-emitting layer is in a range whereconcentration quenching does not occur.

Under the above conditions, energy transfer from the host material tothe guest material is possible even when the T1 level of the hostmaterial is lower than the T1 level of the guest material. Hence, amaterial having a low T1 level can be used as the host material becausethe T1 level of the host material is not necessarily required to behigher than that of the guest material. Additionally, such an elementstructure enables the improvement of reliability of a light-emittingelement compared with that using a material with higher T1 level than aguest material.

$\begin{matrix}{{{E(t)}/E_{0}} = {\sum\limits_{i = 1}^{n}\; {A_{i}{\exp \left( {{- t}/\tau_{i}} \right)}}}} & (1)\end{matrix}$

(where E₀ indicates an initial emission intensity, E(t) indicates anemission intensity at time (t), A is a constant, τ indicates a lifetimeof the each decay component, and n indicates the number of components ofa decay curve.)

Accordingly, one embodiment of the present invention is a light-emittingelement including a light-emitting layer containing at least a hostmaterial and a plurality of guest materials. One of emissions obtainedby irradiating the light-emitting layer with excitation light (theoutput level is set not to cause concentration quenching) shows amulticomponent decay curve and has an emission lifetime more than orequal to 5 μsec and less than or equal to 15 μsec, preferably more thanor equal to 5 μsec and less than or equal to 10 μsec.

Another embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and a light-emitting layer between thepair of electrodes. The light-emitting layer exhibits a plurality ofemissions by photo-excitation. One of the emissions shows amulticomponent decay curve, and an emission lifetime thereof is morethan or equal to 5 μsec and less than or equal to 15 μsec.

Another embodiment of the present invention is a light-emitting elementincluding at least a light-emitting layer between a pair of electrodes.The light-emitting layer contains two kinds of light-emittingsubstances. Emission obtained from one of the light-emitting substancesshows a multicomponent decay curve, and an emission lifetime thereof ismore than or equal to 5 μsec and less than or equal to 15 μsec.

Another embodiment of the present invention is a light-emitting elementincluding at least a light-emitting layer between a pair of electrodes.The light-emitting layer contains at least a first organic compound(host material), a second organic compound (guest material), and a thirdorganic compound (guest material). Each of the second organic compoundand the third organic compound is an organometallic complex. The T1level of the first organic compound is lower than the T1 level of thesecond organic compound. The T1 level of the first organic compound ishigher than the T1 level of the third organic compound. Emissionobtained from the second organic compound shows a multicomponent decaycurve, and an emission lifetime thereof is more than or equal to 5 μsecand less than or equal to 15 μsec.

In each of the above structures, an organic compound whose T1 level islower than that of the guest material can be used as the host material;thus, the light-emitting element can be fabricated without using anorganic compound having low reliability as a host material.

In the structures where the T1 level of the host material is lower thanthe T1 level of the guest material, the difference in T1 level betweenthe host material and the guest material is greater than 0 eV and lessthan or equal to 0.2 eV. Accordingly, a material having high reliabilityas a host material can be used without decreasing emission efficiency,leading to a long-lifetime light-emitting element.

Other embodiments of the present invention are not only a light-emittingdevice including the light-emitting element but also an electronicdevice and a lighting device each including the light-emitting device.Accordingly, a light-emitting device in this specification refers to animage display device or a light source (including a lighting device). Inaddition, the light-emitting device includes, in its category, all of amodule in which a light-emitting device is connected to a connector suchas a flexible printed circuit (FPC) or a tape carrier package (TCP), amodule in which a printed wiring board is provided on the tip of a TCP,and a module in which an integrated circuit (IC) is directly mounted ona light-emitting element by a chip on glass (COG) method.

A light-emitting element of one embodiment of the present invention canhave high emission efficiency. A light-emitting element of oneembodiment of the present invention can have a long lifetime byincluding a material with high reliability as a host material in alight-emitting layer. A light-emitting device of one embodiment of thepresent invention can have high reliability by including thelight-emitting element. An electronic device and a lighting device ofone embodiment of the present invention can have high reliability byincluding the light-emitting device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a concept of one embodiment of the present invention.

FIG. 2 illustrates a structure of a light-emitting element.

FIG. 3 illustrates a structure of a light-emitting element.

FIGS. 4A and 4B illustrate structures of light-emitting elements.

FIGS. 5A and 5B illustrate a light-emitting device.

FIGS. 6A to 6D illustrate electronic devices.

FIGS. 7A to 7C illustrate an electronic device.

FIG. 8 illustrates lighting devices.

FIG. 9 illustrates a structure of a light-emitting element.

FIG. 10 shows luminance versus current density characteristics of alight-emitting element 1 (also referred to as Element 1) and acomparative light-emitting element 2 (also referred to as ReferenceElement 2).

FIG. 11 shows luminance versus voltage characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.

FIG. 12 shows current efficiency versus luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.

FIG. 13 shows current versus voltage characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.

FIG. 14 shows an emission spectrum of each of the light-emitting element1 and the comparative light-emitting element 2.

FIG. 15 shows reliability of each of the light-emitting element 1 andthe comparative light-emitting element 2.

FIG. 16 shows the emission decay curves of light-emitting elements.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description, and modes and details thereof canbe modified in various ways without departing from the spirit and thescope of the present invention. Therefore, the present invention shouldnot be construed as being limited to the description in the followingembodiments.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention is described.

A light-emitting element of one embodiment of the present inventionincludes a light-emitting layer between a pair of electrodes. Thelight-emitting layer contains at least a host material and a pluralityof guest materials. A combination of the host material and one of theguest materials in the light-emitting layer is arranged so that emission(e.g., photoluminescence (PL) by photoexcitation or electroluminescence(EL) by electric field excitation) obtained from the light-emittinglayer shows a multicomponent decay curve and the emission lifetimethereof is short enough to prevail the thermal deactivation process ofthe host material (preferably more than or equal to 5 μsec and less thanor equal to 15 μsec). The light-emitting element with such a structurecan have sufficiently high emission efficiency.

The above-mentioned light-emitting layer is structured so that the T1level of the host material is lower than that of the guest material(hereinafter, referred to as a first guest material for conveniencesake). In the above structure, energy transfer from the host material tothe first guest material is possible even if the T1 level of the hostmaterial is lower than that of the first guest material, and the T1level of the host material is not necessarily required to be higher thanthat of the guest material; thus, a material with high reliability as ahost material can be used. Accordingly, in one embodiment of the presentinvention, the host material whose T1 level is lower than that of thefirst guest material can be used.

The T1 level of another guest material (hereinafter, referred to as asecond guest material for convenience sake) is lower than that of thehost material.

With the application of the above structure, a material with highreliability as a host material in the light-emitting layer can be usedand two or more kinds of lights having different wavelengths can beobtained in the light-emitting layer.

Next, the energy transfer process between the host material and theguest material in the light-emitting layer in the light-emitting elementof one embodiment of the present invention is described with referenceto FIG. 1A.

FIG. 1A illustrates energy levels of excitons of a host material 11 anda first guest material 12 which are included in the light-emittinglayer. T1_((g)) is the triplet excited state of the first guest material12, and T1_((h)) is the triplet excited state of the host material 11that is lower than the T1_((g)) level by ΔE(eV).

In this case, an energy of the T1_((g)) of the first guest material 12transfers (Y_(g)) to the T1^((h)) of the host material 11 at a rate ofk₂×[D*]. Note that [D*] represents the concentration of excitons of thefirst guest material, and k₂ represents a rate constant of energytransfer from the first guest material 12 to the host material 11.Furthermore, energy can transfer (Y_(h)) from the T1_((h)) of the hostmaterial 11 to the T1_((g)) of the first guest material 12 at a rate ofk₃×[H*]. Note that [H*] represents the concentration of excitons of thehost material, and k₃ represents a rate constant of energy transfer fromthe host material 11 to the first guest material 12. In other words, anequilibrium is established between Y_(h) and Y_(g). Thisthermodynamically disadvantageous energy transfer from the low level tothe high level (hereinafter, referred to as reverse energy transfer) canoccur because excitons are activated by energy of room temperature. Notethat in FIG. 1A, k₁ represents a rate constant of transition from theT1_((g)) to an S0_((g)) of the first guest material 12, and k₄represents a rate constant of transition from the T1_((h)) to anS0_((h)) of the host material 11.

Here, the embodiment of the present invention provides a combination ofthe host material 11 and the first guest material 12 so that the energydifference between the T1_((g)) and the T1_((h)) satisfies the formula0<ΔE≦0.2 eV. Therefore, the energy transfer (Y_(g)) from the T1_((g)) tothe T1_((h)) proceeds.

When the above-mentioned energy transfers occur, radiative transition(X_(g)) from the T1_((g)) to the S0_((g)) of the first guest material 12and non-radiative transition (thermal deactivation process X_(h)) fromthe T1_((h)) to the S0_((h)) of the host material 11 also occur at thesame time. At this time, it is also important for highly efficient lightemission that the rate of the radiative transition (X_(g)) be relativelyhigh and the rate of the non-radiative transition (X_(h)) besignificantly low. Specifically, it is preferable that a rate constantk₁ of the radiative transition (X_(g)) be larger than 5.0×10⁵ (sec⁻¹),and a rate constant k₄ of the non-radiative transition (X_(h)) besmaller than 1×10² (sec⁻¹).

That is, the equilibrium between the Y_(h) and Y_(g) is shifted to theside of the first guest by utilizing the rapid radiative transition(X_(g)), slow non-radiative transition (X_(h)), and the small ΔE, whichrealizes the highly efficient light emission.

As described above, since a light-emitting element of one embodiment ofthe present invention also utilizes energy that reversely transfers froma low level for its light emission, the emission from the light-emittinglayer shows a multicomponent decay curve. The lifetime of the emissionis short enough to prevail the thermal deactivation process of the hostmaterial (less than or equal to 15 μsec, preferably less than or equalto 10 μsec, more preferably less than or equal to 5 μsec); thus,sufficiently high emission efficiency can be obtained.

Note that even in the case where a light-emitting layer does notpossesses the aforementioned relation regarding the energy levels, amulticomponent decay curve is obtained when measurement is performed ina state where output of excitation light is set high to provide highconcentration of the exciton. This is because the high excitonconcentration causes the interaction among excitons, leading to thetriplet-triplet extinction. This phenomenon is called concentrationquenching. The measurement is therefore performed in a state whereoutput of excitation light is set low to make the exciton concentrationlow for preventing influence of concentration quenching.

Next, a structure of a light-emitting element of one embodiment of thepresent invention is described with reference to FIG. 2.

As illustrated in FIG. 2, the light-emitting element of one embodimentof the present invention has a structure in which a light-emitting layer104 containing a first organic compound and a second organic compound isprovided between a pair of electrodes (an anode 101 and a cathode 102).The light-emitting layer 104 is one of functional layers included in anEL layer 103 that is in contact with the pair of electrodes. The ELlayer 103 can include, in addition to the light-emitting layer 104, anyof a hole-injection layer, a hole-transport layer, an electron-transportlayer, an electron-injection layer, and the like as appropriate atdesired positions. Note that the light-emitting layer 104 contains atleast a first organic compound 105 serving as the host material, asecond organic compound 106 serving as the first guest material, and athird organic compound 107 serving as the second guest material.

A material having an excellent hole-transport property or a materialhaving an excellent electron-transport property can be used as the firstorganic compound 105 serving as a host material.

Examples of the material having an excellent hole-transport propertythat can be used as the first organic compound 105 include aromaticamine compounds such as4-(1-naphthyl)-4′-phenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBiNB),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N-phenyl-N-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),4,4′-bis(N-{4-[N-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2), and3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2). In addition, the following compounds includinga carbazole skeleton can be used, for example:4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA).The substances mentioned here are mainly ones that have a hole mobilityof 10⁻⁶ cm²/Vs or higher. Note that any substance other than the abovesubstances may be used as long as it has a hole-transport property.

Examples of the material having an excellent electron-transport propertythat can be used as the first organic compound 105 include thefollowings: heterocyclic compounds having polyazole skeletons, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), (abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); heterocyclic compounds having quinoxalineskeletons or dibenzoquinoxaline skeletons, such as2-[3-(dibenzothiophen-4-yl)phenyl] dibenzo[f,h]quinoxaline(abbreviation: 2mDBTPDBq-II),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II), and2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq); heterocyclic compounds having diazineskeletons (pyrimidine skeletons or pyrazine skeletons), such as4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); and heterocyclic compounds having pyridine skeletons, suchas 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy),1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB), and3,3′,5,5′-tetra[(m-pyridyl)-phen-3-yl]biphenyl (abbreviation: BP4 mPy).Among the above-described compounds, the heterocyclic compounds havingquinoxaline skeletons or dibenzoquinoxaline skeletons, the heterocycliccompounds having diazine skeletons, and the heterocyclic compoundshaving pyridine skeletons have high reliability and are thus preferable.Other examples of the material having an excellent electron-transportproperty include the followings: triarylphosphine oxides, such asphenyl-di(1-pyrenyl)phosphine oxide (abbreviation: POPy₂),spiro-9,9′-bifluoren-2-yl-diphenylphosphine oxide (abbreviation: SPPO1),2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (abbreviation: PPT),and 3-(diphenylphosphoryl)-9-[4-(diphenylphosphoryl)phenyl]-9H-carbazole(abbreviation: PPO21); and triarylborane such astris[2,4,6-trimethyl-3-(3-pyridyl)phenyl]borane (abbreviation: 3TPYMB).The substances mentioned here have an electron-transport property andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that any substance other than the above substances may be used aslong as it has an electron-transport property.

Note that the light-emitting layer may contain a fourth organic compoundin addition to the first organic compound (the host material), thesecond organic compound (the first guest material), and the thirdorganic compound (the second guest material). To obtain high emissionefficiency by adjustment of a balance between holes and electrons in thelight-emitting layer, when the first organic compound has ahole-transport property, the fourth organic compound preferably has anelectron-transport property. In contrast, when the first organiccompound has an electron-transport property, the fourth organic compoundpreferably has a hole-transport property. In either case, it ispreferable that the T1_((h)) level of the first organic compound belower than the T1_((g)) level of the second organic compound and higherthan the T1_((g)) level of the third organic compound. However, in thecase where the T1 level of the fourth organic compound is lower than theT1_((g)) level of the first organic compound, the above-mentionedinteraction between a host material and a guest material occurs betweenthe fourth organic compound and the second organic compound, and betweenthe fourth organic compound and the third organic compound. Accordingly,it is preferable that the T1 level of the fourth organic compound belower than that of the second organic compound and higher than that ofthe third organic compound. This is because when two or more materialsother than a light-emitting guest material exist, energy preferentiallytransfers therebetween from higher T1 to lower T1.

As the second organic compound 106 and the third organic compound 107which serve as guest materials, an organometallic complex (aphosphorescent compound) that is a light-emitting substance convertingtriplet excitation energy into light emission can be used, for example.

Examples of the material that can be used as the second organic compound106 and the third organic compound 107 includebis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: Flrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: [Ir(CF₃ ppy)₂(pic)]),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac),tris(2-phenylpyridinato)iridium(III) (abbreviation: [Ir(ppy)₃]),bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation:[Ir(ppy)₂(acac)]), bis(benzo[h]quinolinato)iridium(III) acetylacetonate(abbreviation: [Ir(bzq)₂(acac)]),bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: [Ir(dpo)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate (abbreviation: [Ir(p-PF-ph)₂(acac)]),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(bt)₂(acac)]),bis[2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate(abbreviation: [Ir(btp)₂(acac)]),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(piq)₂(acac)]),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: [Ir(Fdpq)₂(acac)]),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphineplatinum(II)(abbreviation: PtOEP),tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)).

The host material and the first guest material contained in thelight-emitting layer of the light-emitting element described in thisembodiment satisfy following conditions: the emission (e.g.,photoluminescence (PL) by photoexcitation or electroluminescence (EL) byelectric field excitation) obtained from their combination shows amulticomponent decay curve; and the lifetime thereof is short enough toprevail the thermal deactivation process of the host material (more thanor equal to 5 μsec and less than or equal to 15 μsec, preferably morethan or equal to 5 μsec and less than or equal to 10 μsec). Thisstructure provides sufficiently high emission efficiency. Furthermore,in the light-emitting layer, the third organic compound 107 (the secondguest material) can emit light using the same host material. Since thehost material is used in common for different guest materials, anintermolecular carrier-injection barrier between the host materials isnot formed; as a result, a plurality of lights having differentwavelengths can be obtained from the light-emitting layer withoutincreasing driving voltage.

In the light-emitting element in this embodiment, the T1 level of thehost material (the first organic compound 105) is lower than the T1level of the first guest material (the second organic compound 106), andhigher than the T1 level of the second guest material (the third organiccompound 107). The emission from a plurality of guest materials in thesame host material is resulted from energy transfer to the secondorganic compound 106 serving as first a guest material, which occurseven when the T1 level of the first organic compound 105 is lower thanthat of the second organic compound 106. Hence, the T1 level of the hostmaterial can be lower than that of a generally-used host material, whichallows the use of a material with high reliability as a host material.Therefore, the light-emitting element can have a long lifetime.

The structure of this embodiment exhibits the delayed light emissionassociated with reverse energy transfer between the host material andthe first guest material whose T1 level is higher than that of the hostmaterial. Since the host material in the T1 is non-radiative at roomtemperature, it is concerned that a light-emitting layer exhibitingdelayed light emission causes low efficiency. However, the reverseenergy transfer is allowed due to the aforementioned small difference inT1 level, and the rate of the transition (radiative deactivation rate)of the guest material is sufficiently higher than the rate of thetransition (non-radiative deactivation rate) of the host material; thus,element characteristics are not affected and a light-emitting elementhaving high emission efficiency can be obtained.

Embodiment 2

In this embodiment, an example of a light-emitting element of oneembodiment of the present invention is described with reference to FIG.3.

In the light-emitting element described in this embodiment, asillustrated in FIG. 3, an EL layer 203 including a light-emitting layer206 is provided between a pair of electrodes (a first electrode (anode)201 and a second electrode (cathode) 202), and the EL layer 203 includesa hole-injection layer 204, a hole-transport layer 205, anelectron-transport layer 207, an electron-injection layer 208, and thelike in addition to the light-emitting layer 206.

As in the light-emitting element described in Embodiment 1, thelight-emitting layer 206 contains at least the first organic compoundserving as the host material, the second organic compound serving as thefirst guest material, and the third organic compound serving as thesecond guest material. Since the same substances described in Embodiment1 can be used as the first organic compound, the second organiccompound, and the third organic compound, description thereof isomitted.

In addition to the first organic compound serving as the host material,the second organic compound serving as the first guest material, and thethird organic compound serving as the second guest material, thelight-emitting layer 206 may also contain the fourth organic compoundhaving a carrier-transport property opposite to that of the firstorganic compound (a hole-transport property or an electron-transportproperty).

Next, a specific example in manufacturing the light-emitting elementdescribed in this embodiment is described.

For the first electrode (anode) 201 and the second electrode (cathode)202, a metal, an alloy, an electrically conductive compound, a mixturethereof, or the like can be used. Specifically, indium oxide-tin oxide(ITO: indium tin oxide), indium oxide-tin oxide containing silicon orsilicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used.In addition, an element belonging to Group 1 or Group 2 of the periodictable, for example, an alkali metal such as lithium (Li) or cesium (Cs),an alkaline earth metal such as magnesium (Mg), calcium (Ca), orstrontium (Sr), an alloy containing such an element (e.g., MgAg orAlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), analloy containing such an element, or graphene can be used. The firstelectrode (anode) 201 and the second electrode (cathode) 202 can beformed by, for example, a sputtering method or an evaporation method(including a vacuum evaporation method).

Examples of a material having an excellent hole-transport property thatcan be used for the hole-injection layer 204 and the hole-transportlayer 205 include aromatic amine compounds such as NPB,TPD, TCTA, TDATA,MTDATA, BSPB; and 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP); carbazole derivatives such as PCzPCA1, PCzPCA2,and PCzPCN1), CBP, TCPB, and CzPA; and dibenzothiophene derivatives suchas 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II). Thesubstances mentioned here are mainly materials having a hole mobility of10⁻⁶ cm²/Vs or higher. Note that substances other than the abovesubstances may be used as long as the hole-transport property is higherthan the electron-transport property.

Alternatively, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can be used.

The hole-injection layer 204 may be doped with a substance serving as anelectron acceptor (acceptor) to the substance having a highhole-transport property. As examples of the acceptor, an oxide of ametal belonging to any of Group 4 to Group 8 of the periodic table canbe given. Specifically, molybdenum oxide is particularly preferable.

The light-emitting layer 206 contains, as described above, at least thefirst organic compound 209 serving as the host material, the secondorganic compound 210 serving as the first guest material, and the thirdorganic compound 211 serving as the second guest material.

Note that for the hole-transport layer 205 in contact with thelight-emitting layer 206, a compound similar to the organic compoundcontained in the light-emitting layer is preferably used. With thisstructure, the hole-injection barrier between the hole transport layer205 and the light-emitting layer 206 can be reduced, which can increaseemission efficiency and reduce driving voltage. That is, alight-emitting element having a small decrease in power efficiency dueto voltage loss can be obtained. A particularly preferable mode forreducing the hole-injection barrier is a structure in which thehole-transport layer 205 contains the first organic compound as thelight-emitting layer.

The electron-transport layer 207 is a layer containing a material havingan excellent electron-transport property. For the electron-transportlayer 207, a metal complex such as tris(8-quinolinolato)aluminum(III)(abbreviation: Alq₃), tris(4-methyl-8-quinolinolato)aluminum(III)(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis[2-2 (hydroxyphenyl)benzoxazolato]zinc (II)(abbreviation: Zn(BOX)₂), orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II) (abbreviation: Zn(BTZ)₂)can be used. Further, a heteroaromatic compound such as PBD, OXD-7, TAZ,3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can alsobe used. A high molecular compound such as poly(2,5-pyridinediyl)(abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation:PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances given here are mainlyones having an electron mobility of 10⁻⁶ cm²/Vs or higher. Note that anysubstance other than the above substances may be used for theelectron-transport layer 207 as long as the electron-transport propertyis higher than the hole-transport property.

The electron-transport layer 207 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

The electron-injection layer 208 is a layer containing a substancehaving a high electron-injection property. For the electron-injectionlayer 208, an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiO_(x)), can be used. A rare earthmetal compound like erbium fluoride (ErF₃) can also be used. Any of theabove substances for forming the electron-transport layer 207 can alsobe used.

A composite material in which an organic compound and an electron donor(donor) are mixed may also be used for the electron-injection layer 208.Such a composite material is excellent in an electron-injection propertyand an electron-transport property because electrons are generated inthe organic compound by the electron donor. In this case, the organiccompound is preferably a material excellent in transporting thegenerated electrons. Specifically, for example, the above materials forforming the electron-transport layer 207 (e.g., a metal complex or aheteroaromatic compound) can be used. As the electron donor, a substanceexhibiting an electron-donating property with respect to the organiccompound may be used. Specifically, an alkali metal, an alkaline earthmetal, and a rare earth metal are preferable, and lithium, cesium,magnesium, calcium, erbium, and ytterbium are exemplified. Further, analkali metal oxide or an alkaline earth metal oxide is preferable, andfor example, lithium oxide, calcium oxide, and barium oxide can be used.A Lewis base such as magnesium oxide can also be used. An organiccompound such as tetrathiafulvalene (abbreviation: TTF) can also beused.

Note that each of the above hole-injection layer 204, hole-transportlayer 205, light-emitting layer 206, electron-transport layer 207, andelectron-injection layer 208 can be formed by, for example, anevaporation method (e.g., a vacuum evaporation method), an inkjetmethod, or a coating method.

Light emission obtained in the light-emitting layer 206 of theabove-described light-emitting element is extracted to the outsidethrough either the first electrode 201 or the second electrode 202 orboth. Therefore, either the first electrode 201 or the second electrode202 in this embodiment, or both, is an electrode having alight-transmitting property.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 3

In this embodiment, as one embodiment of the present invention, alight-emitting element (hereinafter referred to as tandem light-emittingelement) in which a charge generation layer is provided between aplurality of EL layers is described.

The light-emitting element described in this embodiment is a tandemlight-emitting element including a plurality of EL layers (a first ELlayer 302(1) and a second EL layer 302(2)) between a pair of electrodes(a first electrode 301 and a second electrode 304) as illustrated inFIG. 4A.

In this embodiment, the first electrode 301 functions as an anode, andthe second electrode 304 functions as a cathode. Note that the firstelectrode 301 and the second electrode 304 can have structures similarto those described in Embodiment 2. In addition, all or any of theplurality of EL layers (the first EL layer 302(1) and the second ELlayer 302(2)) may have structures similar to those described inEmbodiment 2. In other words, the structures of the first EL layer302(1) and the second EL layer 302(2) may be the same or different fromeach other and can be similar to those of the EL layers described inEmbodiment 2.

A charge generation layer 305 is provided between the plurality of ELlayers (the first EL layer 302(1) and the second EL layer 302(2)). Thecharge-generation layer 305 has a function of injecting electrons intoone of the EL layers and injecting holes into the other of the EL layerswhen voltage is applied between the first electrode 301 and the secondelectrode 304. In this embodiment, when voltage is applied such that thepotential of the first electrode 301 is higher than that of the secondelectrode 304, the charge-generation layer 305 injects electrons intothe first EL layer 302(1) and injects holes into the second EL layer302(2).

Note that for improving light extraction efficiency, thecharge-generation layer 305 preferably has a property of transmittingvisible light (specifically, the charge-generation layer 305 preferablyhas a visible light transmittance of 40% or higher). Further, thecharge-generation layer 305 functions even when it has lowerconductivity than the first electrode 301 or the second electrode 304.

The charge-generation layer 305 may have either a structure in which anelectron acceptor (acceptor) is added to an organic compound having anexcellent hole-transport property or a structure in which an electrondonor (donor) is added to an organic compound having an excellentelectron-transport property. Alternatively, both of these structures maybe stacked.

In the case of the structure in which an electron acceptor is added toan organic compound having an excellent hole-transport property, as theorganic compound having an excellent hole-transport property, forexample, an aromatic amine compound such as NPB, TPD, TDATA, MTDATA, orBSPB can be used. The substances given here are mainly ones having ahole mobility of 10⁻⁶ cm²/Vs or higher. However, any substance otherthan the above substances may be used as long the hole-transportproperty is higher than the electron-transport property.

Examples of the electron acceptor include a halogen compound such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F4TCNQ) or chloranil; and a cyano compound such aspyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (abbreviation:PPDN) ordipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(abbreviation: HAT-CN). Examples of the electron acceptor also includeoxides of metals that belong to Group 4 to Group 8 of the periodictable. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperty. Among these, molybdenum oxide is especially preferable becauseit is stable in the air, has a low hygroscopic property, and is easilyhandled.

In the case of the structure in which an electron donor is added to anorganic compound having an excellent electron-transport property, as theorganic compound having an excellent electron-transport property, forexample, a metal complex having a quinoline skeleton or a benzoquinolineskeleton, such as Alq, Almq₃, BeBq₂, or BAlq, can be used. A metalcomplex having an oxazole-based ligand or a thiazole-based ligand, suchas Zn(BOX)₂ or Zn(BTZ)₂, or the like can also be used. Other than metalcomplexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. Thesubstances given here are mainly ones having an electron mobility of10⁻⁶ cm²/Vs or higher. Note that substances other than the abovesubstances may be used as long as the electron-transport property ishigher than the hole-transport property.

Further, as the electron donor, an alkali metal, an alkaline earthmetal, a rare earth metal, a metal belonging to Group 2 or Group 13 ofthe periodic table, or an oxide or carbonate thereof can be used.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. An organic compound such astetrathianaphthacene may be also used as the electron donor.

Note that formation of the charge-generation layer 305 with use of anyof the above materials can suppress an unnecessary increase in drivevoltage caused by the stack of the EL layers.

Although the light-emitting element having two EL layers is described inthis embodiment, the present invention can be similarly applied to alight-emitting element in which n EL layers are stacked as illustratedin FIG. 4B. In the case where a plurality of EL layers is providedbetween a pair of electrodes as in the light-emitting element of thisembodiment, by providing the charge-generation layer between the ELlayers, the light-emitting element can emit light in a high luminanceregion while the current density is kept low. Since the current densitycan be kept low, the element can have a long lifetime. When thelight-emitting element is applied to light-emitting devices, electronicapparatus, and lighting devices each having a large light-emitting area,voltage drop due to resistance of an electrode material can be reduced,thereby achieving homogeneous light emission in the whole of thelight-emitting area.

By making emission colors of the EL layers different, light of a desiredcolor can be obtained from the light-emitting element as a whole. Forexample, the emission colors of first and second EL layers arecomplementary in a light-emitting element having the two EL layers,whereby the light-emitting element can emit white light as a whole. Notethat the term “complementary” means color relationship in which anachromatic color is obtained when colors are mixed. In other words,emission of white light can be obtained by mixture of light emitted fromsubstances whose emission colors are complementary colors.

Further, the same applies to a light-emitting element having three ELlayers. For example, the light-emitting element as a whole can emitwhite light when the emission color of the first EL layer is red, theemission color of the second EL layer is green, and the emission colorof the third EL layer is blue.

As well as the structure described in this embodiment in which the ELlayers are stacked with the charge generation layer providedtherebetween, the light-emitting element may have a micro opticalresonator (microcavity) structure which utilizes a light resonant effectby adjusting a distance between the electrodes (the first electrode 301and the second electrode 304) to a desired value.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 4

In this embodiment, a light-emitting device including a light-emittingelement of one embodiment of the present invention is described.

Note that any of the light-emitting elements described in the otherembodiments can be used as the light-emitting element. Either a passivematrix light-emitting device or an active matrix light-emitting devicemay be used as the light-emitting device. An active matrixlight-emitting device is described in this embodiment with reference toFIGS. 5A and 5B.

Note that FIG. 5A is a top view illustrating a light-emitting device andFIG. 5B is a cross-sectional view taken along the chain line A-A′ inFIG. 5A. The active matrix light-emitting device of this embodimentincludes a pixel portion 502 provided over an element substrate 501, adriver circuit portion (a source line driver circuit) 503, and drivercircuit portions (gate line driver circuits) 504 a and 504 b. The pixelportion 502, the driver circuit portion 503, and the driver circuitportions 504 a and 504 b are sealed between the element substrate 501and a sealing substrate 506 with a sealant 505.

A lead wiring 507 is provided over the element substrate 501. The leadwiring 507 is provided for connecting an external input terminal throughwhich a signal (e.g., a video signal, a clock signal, a start signal,and a reset signal) or a potential from the outside is transmitted tothe driver circuit portion 503 and the driver circuit portions 504 a and504 b. Here is shown an example in which a flexible printed circuit(FPC) 508 is provided as the external input terminal. Although the FPCis illustrated alone, this FPC may be provided with a printed wiringboard (PWB). The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.5B. The driver circuit portion and the pixel portion are formed over theelement substrate 501; here are illustrated the driver circuit portion503 which is the source line driver circuit and the pixel portion 502.

The driver circuit portion 503 is an example where a CMOS circuit isformed, which is a combination of an n-channel FET 509 and a p-channelFET 510. Note that a circuit included in the driver circuit portion maybe formed using various CMOS circuits, PMOS circuits, or NMOS circuits,and any of a staggered type FET and a reverse-staggered type FET can beused. Further, the crystallinity of a semiconductor film used in the FETis not limited and can be amorphous or crystalline. Additionally, anoxide semiconductor may be used for the semiconductor film. Examples ofthe semiconductor material include element semiconductors such assilicon, germanium, tin, selenium, and tellurium; compoundsemiconductors such as GaAs, GaP, InSb, ZnS, and CdS; and oxidesemiconductors such as SnO₂, ZnO, Fe₂O₃, V₂O₅, TiO₂, NiO, Cr₂O₃, Cu₂O,MnO₂, MnO, and InGaZnO (including the ones having different atomicratios). Although this embodiment shows a driver integrated type inwhich the driver circuit is formed over the substrate, the drivercircuit is not necessarily formed over the substrate, and may be formedoutside the substrate.

The pixel portion 502 is formed of a plurality of pixels each of whichincludes a switching FET 511, a current control FET 512, and a firstelectrode (anode) 513 which is electrically connected to a wiring (asource electrode or a drain electrode) of the current control FET 512.Note that an insulator 514 is formed to cover end portions of the firstelectrode (anode) 513. In this embodiment, the insulator 514 is formedusing a positive photosensitive acrylic resin.

The insulator 514 preferably has a curved surface with curvature at anupper end portion or a lower end portion thereof in order to obtainfavorable coverage by a film which is to be stacked over the insulator514. For example, in the case of using a positive photosensitive acrylicresin as a material of the insulator 514, the insulator 514 preferablyhas a curved surface with a curvature radius (0.2 μm to 3 μm) at theupper end portion. Note that the insulator 514 can be formed usingeither a negative photosensitive resin or a positive photosensitiveresin. The material of the insulator 514 is not limited to an organiccompound, and an inorganic compound such as silicon oxide or siliconoxynitride can also be used.

An EL layer 515 and a second electrode (cathode) 516 are stacked overthe first electrode (anode) 513, so that a light-emitting element 517 isformed. Note that the EL layer 515 includes at least the light-emittinglayer described in Embodiment 1. In the EL layer 515, a hole-injectionlayer, a hole-transport layer, an electron-transport layer, anelectron-injection layer, a charge-generation layer, and the like can beprovided as appropriate in addition to the light-emitting layer.

For the first electrode (anode) 513, the EL layer 515, and the secondelectrode (cathode) 516, the materials described in Embodiment 2 can beused. Although not illustrated, the second electrode (cathode) 516 iselectrically connected to the FPC 508 which is an external inputterminal

Although the cross-sectional view of FIG. 5B illustrates only onelight-emitting element 517, a plurality of light-emitting elements isarranged in a matrix in the pixel portion 502. Light-emitting elementswhich provide three kinds of light emission (R, G, and B) areselectively formed in the pixel portion 502, whereby a light-emittingdevice capable of full color display can be fabricated. Alternatively, alight-emitting device which is capable of full color display may befabricated by a combination with color filters.

Further, the sealing substrate 506 is attached to the element substrate501 with the sealant 505, whereby the light-emitting element 517 isprovided in a space 518 surrounded by the element substrate 501, thesealing substrate 506, and the sealant 505. The space 518 may be filledwith an inert gas (such as nitrogen or argon), or the sealant 505.

An epoxy-based resin or a glass frit is preferably used for the sealant505. It is preferable that such a material allow permeation of moistureor oxygen as little as possible. As the sealing substrate 506, a glasssubstrate, a quartz substrate, or a plastic substrate formed offiber-reinforced plastic (FRP), polyvinyl fluoride) (PVF), a polyester,an acrylic resin, or the like can be used. In the case where glass fritis used as the sealant, the element substrate 501 and the sealingsubstrate 506 are preferably glass substrates.

As described above, an active matrix light-emitting device can beobtained.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 5

In this embodiment, examples of a variety of electronic devices whichare completed using a light-emitting device are described with referenceto FIGS. 6A to 6D and FIGS. 7A to 7C. The light-emitting device isfabricated using the light-emitting element of one embodiment of thepresent invention.

Examples of the electronic devices to which the light-emitting device isapplied include television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, digital cameras,digital video cameras, digital photo frames, mobile phones (alsoreferred to as cellular phones or mobile phone devices), portable gamemachines, portable information terminals, audio playback devices, andlarge-sized game machines such as pin-ball machines. Specific examplesof the electronic devices are illustrated in FIGS. 6A to 6D.

FIG. 6A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.Images can be displayed by the display portion 7103, and thelight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the receiver, a general television broadcastcan be received. Furthermore, when the television device 7100 isconnected to a communication network by wired or wireless connection viathe modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

FIG. 6B illustrates a computer including a main body 7201, a housing7202, a display portion 7203, a keyboard 7204, an external connectingport 7205, a pointing device 7206, and the like. This computer ismanufactured by using the light-emitting device of one embodiment of thepresent invention for the display portion 7203.

FIG. 6C illustrates a portable game machine including two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.A display portion 7304 is incorporated in the housing 7301 and a displayportion 7305 is incorporated in the housing 7302. In addition, theportable game machine illustrated in FIG. 6C includes a speaker portion7306, a recording medium insertion portion 7307, an LED lamp 7308, aninput means (an operation key 7309, a connection terminal 7310, and amicrophone 7312), a sensor 7311 (a sensor having a function of measuringor sensing force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,current, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), and the like. It isneedless to say that the structure of the portable game machine is notlimited to the above as long as a light-emitting device is used for atleast either the display portion 7304 or the display portion 7305, orboth, and may include other accessories as appropriate. The portablegame machine illustrated in FIG. 6C has a function of reading out aprogram or data stored in a storage medium to display it on the displayportion, and a function of sharing information with another portablegame machine by wireless communication. The portable game machineillustrated in FIG. 6C can have a variety of functions withoutlimitation to the above.

FIG. 6D illustrates an example of a mobile phone. A mobile phone 7400 isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using a light-emitting device for the display portion7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 6D is touched with a finger or the like, data can be input into themobile phone 7400. Further, operations such as making a call andcomposing an e-mail can be performed by touch on the display portion7402 with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device including a sensor (e.g., a gyroscope or anacceleration sensor) is provided inside the mobile phone 7400, displayon the screen of the display portion 7402 can be automatically changedby determining the orientation of the mobile phone 7400 (whether themobile phone is placed horizontally or vertically).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on the kind of imagesdisplayed on the display portion 7402. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when it is determined that input bytouching the display portion 7402 is not performed within a specifiedperiod on the basis of a signal detected by an optical sensor in thedisplay portion 7402, the screen mode may be controlled so as to beswitched from the input mode to the display mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing asensing light source which emits a near-infrared light in the displayportion, an image of a finger vein, a palm vein, or the like can betaken.

FIGS. 7A and 7B illustrate a foldable tablet terminal. In FIG. 7A, astate that the tablet terminal is opened is illustrated. The tabletterminal includes a housing 9630, a display portion 9631 a, a displayportion 9631 b, a display mode switch 9034, a power switch 9035, a powersaver switch 9036, a clasp 9033, and an operation switch 9038. Thetablet terminal is manufactured using the light-emitting device for oneor both of the display portions 9631 a and 9631 b.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9637 that aredisplayed. Note that FIG. 7A shows, as an example, that half of the areaof the display portion 9631 a has only a display function and the otherhalf of the area has a touch panel function. However, the structure ofthe display portion 9631 a is not limited to this mode, and all the areaof the display portion 9631 a may have a touch panel function. Forexample, all the area of the display portion 9631 a can display keyboardbuttons and serve as a touch panel while the display portion 9631 b canbe used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Furthermore, touch input can be performed concurrently on the touchpanel regions 9632 a and 9632 b.

The switch 9034 for switching display modes can switch displayorientation (e.g., between landscape mode and portrait mode) and selecta display mode (switch between monochrome display and color display),for example. With the switch 9036 for switching to power-saving mode,the luminance of display can be optimized in accordance with the amountof external light at the time when the tablet terminal is in use, whichis measured with an optical sensor incorporated in the tablet terminal.The tablet terminal may include another detection device such as asensor (e.g., a gyroscope or an acceleration sensor) in addition to theoptical sensor.

Although FIG. 7A shows the example where the display area of the displayportion 9631 a is the same as that of the display portion 9631 b, oneembodiment of the present invention is not limited to this example. Theymay differ in size and/or image quality. For example, one of them may bea display panel that can display higher-definition images than theother.

FIG. 7B illustrates the tablet terminal which is closed. The tabletterminal includes the housing 9630, a solar battery 9633, acharge/discharge control circuit 9634, a battery 9635, and a DC to DCconverter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing a tabletterminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 7A and 7B can also have afunction of displaying various kinds of data, such as a calendar, adate, or the time, on the display portion as a still image, a movingimage, and a text image, a function of displaying, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630 andthus the battery 9635 can be charged efficiently. When a lithium ionbattery is used as the battery 9635, there is an advantage of downsizingor the like.

The structure and operation of the charge/discharge control circuit 9634illustrated in FIG. 7B are described with reference to a block diagramin FIG. 7C. FIG. 7C illustrates the solar battery 9633, the battery9635, the DC to DC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DC to DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to those in the charge/discharge control circuit 9634illustrated in FIG. 7B.

An example of the operation performed when power is generated by thesolar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by the DCto DC converter 9636 so as to be a voltage for charging the battery9635. Then, when power from the solar battery 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9638 soas to be a voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

It is needless to say that an embodiment of the present invention is notlimited to the electronic device illustrated in FIGS. 7A to 7C as longas the display portion described in the above embodiment is included.

As described above, the electronic devices can be obtained byapplication of the light-emitting device of one embodiment of thepresent invention. The light-emitting device has an extremely wideapplication range, and can be applied to electronic devices in a varietyof fields.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 6

In this embodiment, examples of lighting devices which are completedusing a light-emitting device are described with reference to FIG. 8.The light-emitting device is fabricated using a light-emitting elementof one embodiment of the present invention.

FIG. 8 illustrates an example in which the light-emitting device is usedas an indoor lighting device 8001. Since the light-emitting device canhave a larger area, it can be used for a lighting device having a largearea. In addition, a lighting device 8002 in which a light-emittingregion has a curved surface can also be obtained with the use of ahousing with a curved surface. A light-emitting element included in thelight-emitting device described in this embodiment is in a thin filmform, which allows the housing to be designed more freely. Therefore,the lighting device can be elaborately designed in a variety of ways.Further, a wall of the room may be provided with a large-sized lightingdevice 8003.

Moreover, when the light-emitting device is used at a surface of atable, a lighting device 8004 which has a function as a table can beobtained. When the light-emitting device is used as part of otherfurniture, a lighting device which has a function as the furniture canbe obtained.

As described above, a variety of lighting devices to which thelight-emitting device is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Example 1

In this example, a light-emitting element 1 (Element 1) and acomparative light-emitting element 2 (Reference Element 2) which areembodiments of the present invention are described with reference toFIG. 9. Chemical formulae of materials used in this example are shownbelow.

<<Fabrication of Light-Emitting Element 1 and Comparative Light-EmittingElement 2>>

First, a film of indium oxide-tin oxide containing silicon oxide (ITSO)was formed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. The thicknesswas 110 nm and the electrode area was 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, the surface of the substrate was washed with water,baked at 200° C. for 1 hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus in which the pressure had been reduced to approximately 10⁻⁴Pa, and subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Then, the substrate 1100 over which the first electrode 1101 was formedwas fixed to a substrate holder provided in the vacuum evaporationapparatus so that the surface provided with the first electrode 1101faced downward. In this example, a case is described in which ahole-injection layer 1111, a hole-transport layer 1112, a light-emittinglayer 1113, an electron-transport layer 1114, and an electron-injectionlayer 1115 which are included in an EL layer 1102 are sequentiallyformed by a vacuum evaporation method.

After reducing the pressure in the vacuum evaporation apparatus to 10⁻⁴Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum(VI) oxide were co-evaporated with a mass ratio of DBT3P-II tomolybdenum oxide being 2:1, whereby the hole-injection layer 1111 wasformed over the first electrode 1101. The thickness was 33 nm. Note thata co-evaporation method is an evaporation method in which a plurality ofdifferent substances is concurrently vaporized from respective differentevaporation sources.

Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was evaporated to a thickness of 20 nm, so that thehole-transport layer 1112 was formed.

Next, the light-emitting layer 1113 (a first light-emitting layer 1113 aand a second light-emitting layer 1113 b) was formed over thehole-transport layer 1112. For the light-emitting element 1, thelight-emitting layer 1113 having a stacked structure was formed asfollows:2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),4-(1-naphthyl)-4′-phenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBiNB), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) were co-evaporated to a thickness of20 nm with a mass ratio of 2mDBTBPDBq-II to PCBBiNB and[Ir(tBuppm)₂(acac)] being 0.8:0.2:0.06, whereby the first light-emittinglayer 1113 a was formed; co-evaporation was further performed usingbis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]) instead of [Ir(tBuppm)₂(acac)] to athickness of 20 nm with a mass ratio of 2mDBTBPDBq-II to PCBBiNB and[Ir(tppr)₂(dpm)] being 0.9:1.0:0.06, whereby the second light-emittinglayer 1113 b was formed.

For the comparative light-emitting element 2, 2mDBTBPDBq-II,4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), and [Ir(tBuppm)₂(acac)] were co-evaporated to a thickness of20 nm with a mass ratio of 2mDBTBPDBq-II to PCBA1BP and[Ir(tBuppm)₂(acac)] being 0.8:0.2:0.06, and then further co-evaporatedto a thickness of 20 nm with a mass ratio of 2mDBTBPDBq-II to PCBA1BPand [Ir(tppr)₂(dpm)] being 0.9:0.1:0.06; thus, the light-emitting layer1113 was formed.

Then, 2mDBTBPDBq-II was evaporated to a thickness of 15 nm over thelight-emitting layer 1113 and bathophenanthroline (abbreviation: Bphen)was evaporated to a thickness of 15 nm, whereby the electron-transportlayer 1114 having a stacked structure was formed. Furthermore, lithiumfluoride was evaporated to a thickness of 1 nm over theelectron-transport layer 1114, whereby the electron-injection layer 1115was formed.

Finally, aluminum was evaporated to a thickness of 200 nm over theelectron-injection layer 1115 to form a second electrode 1103 serving asa cathode; thus, the light-emitting element 1 and the comparativelight-emitting element 2 were obtained. Note that, in the aboveevaporation process, evaporation was all performed by a resistanceheating method.

In the above-described manner, the light-emitting element 1 and thecomparative light-emitting element 2 were obtained. Table 1 showselement structures of the light-emitting element 1 (Element 1) and thecomparative light-emitting element 2 (Reference Element 2).

TABLE 1 Light-emitting First layer Second electrode HIL^(a) HTL^(b) 1st2nd ETL^(c) EIL^(d) electrode Element 1 ITSO DBT3P-II:MoOx BPAFLP ^(e)^(f) 2mDBTBPDBq-II Bphen LiF Al Reference (110 nm) (2:1 33 nm) (20 nm)^(g) ^(h) (15 nm) (15 nm) (1 nm) (200 nm) Element 2 ^(a)Hole-injectionlayer. ^(b)Hole-transport layer. ^(c)Electron-transport layer.^(d)Electron-injection layer. ^(e)2mDBTBPDBq-II:PCBBiNB:[Ir(tBuppm)₂(acac)] (0.8:0.2:0.06 20 nm). ^(f)2mDBTBPDBq-II:PCBBiNB:[Ir(tppr)₂(dpm)] (0.9:0.1:0.06 20 nm) ^(g)2mDBTBPDBq-II:PCBA1BP:[Ir(tBuppm)₂(acac)] (0.8:0.2:0.06 20 nm) ^(h)2mDBTBPDBq-II:PCBA1BP:Ir(tppr)₂(dpm)] (0.9:0.1:0.06 20 nm)

The fabricated light-emitting element 1 and comparative light-emittingelement 2 were sealed in a glove box containing a nitrogen atmosphere soas not to be exposed to the air (specifically, a sealant was appliedonto outer edges of the elements and heat treatment was performed at 80°C. for 1 hour at the time of sealing).

<<Operation Characteristics of Light-Emitting Element 1 and ComparativeLight-Emitting Element 2>>

Operation characteristics of the fabricated light-emitting element 1 andcomparative light-emitting element 2 were measured. Note that themeasurement was carried out at room temperature (in an atmosphere keptat 25° C.).

FIG. 10 shows luminance versus current density characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.In FIG. 10, the vertical axis represents luminance (cd/m²) and thehorizontal axis represents current density (mA/cm²). FIG. 11 showsluminance versus voltage characteristics of the light-emitting element 1and the comparative light-emitting element 2. In FIG. 11, the verticalaxis represents luminance (cd/m²) and the horizontal axis representsvoltage (V). FIG. 12 shows current efficiency versus luminancecharacteristics of the light-emitting element 1 and the comparativelight-emitting element 2. In FIG. 12, the vertical axis representscurrent efficiency (cd/A) and the horizontal axis represents luminance(cd/m²). FIG. 13 shows current versus voltage characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.In FIG. 13, the vertical axis represents current (mA) and the horizontalaxis represents voltage (V).

The results of FIG. 10, FIG. 11, FIG. 12, and FIG. 13 reveal that thelight-emitting element 1 of one embodiment of the present invention andthe comparative light-emitting element 2 show favorable characteristics(see Table 2 given below) and no large difference was observedtherebetween although in a light-emitting layer of the light-emittingelement 1, PCBBiNB whose T1 level is lower than that of[Ir(tBuppm)₂(acac)] is used as a host material and [Ir(tBuppm)₂(acac)]is used as a guest material while in a light-emitting layer of thecomparative light-emitting element 2, PCBA1BP whose T1 level is higherthan that of [Ir(tBuppm)₂(acac)] is used as a host material.

Table 2 shows initial performance of main characteristics of thelight-emitting element 1 and the comparative light-emitting element 2 ata luminance of about 1000 cd/m².

TABLE 2 Current Current Power External Voltage Current densityChromaticity Luminance efficiency efficiency quantum (V) (mA) (mA/cm²)(x, y) (cd/m²) (cd/A) (1m/W) efficiency (%) Element 1 3.4 0.12 3.1(0.49, 0.50) 900 29 27 14 Reference 3.5 0.17 4.2 (0.52, 0.47) 1000 24 2114 Element 2

The above results in Table 2 also show that each of the light-emittingelement 1 and the comparative light-emitting element 2 fabricated inthis example has high quantum efficiency.

FIG. 14 shows emission spectra of the light-emitting element 1 and thecomparative light-emitting element 2 which were obtained when a currentof 0.1 mA flowed in these light-emitting elements. As shown in FIG. 14,the emission spectrum of the light-emitting element 1 has peaks at 546nm and 620 nm, which are found to be derived from emission of[Ir(tBuppm)₂(acac)] and [Ir(tppr)₂(dpm)], respectively, contained in thelight-emitting layer 1113. The comparative light-emitting element 2 alsohas peaks derived from emission of [Ir(tBuppm)₂(acac)] and[Ir(tppr)₂(dpm)], but the emission intensity at 546 nm is lower thanthat of the light-emitting element 1. This is probably becauselight-emitting regions differ depending on materials used for thelight-emitting elements.

Next, reliability tests of the light-emitting element 1 and thecomparative light-emitting element 2 were conducted. FIG. 15 showsresults of the reliability tests. In FIG. 15, the vertical axisrepresents normalized luminance (%) with an initial luminance of 100%,and the horizontal axis represents driving time (h) of the elements.Note that in the reliability tests, the light-emitting element 1 and thecomparative light-emitting element 2 were driven under the conditionsthat the initial luminance was 5000 cd/m² and the current density wasconstant. As a result, the luminance of the light-emitting element 1after 100-hour driving was about 85% of the initial luminance; thus, thelight-emitting element 1 kept higher luminance than that of thecomparative light-emitting element 2.

The above reliability tests show that the light-emitting element 1 ofone embodiment of the present invention has high reliability and a longlifetime.

Example 2

In this example, several samples in which an organic film (thickness: 50nm) was provided between quartz substrates were fabricated, and thelifetime (τ₁, τ₂) [μsec] of each sample was measured. The organic filmwas made using 2mDBTBPDBq-II (host material), PCBBiNB or PCBA1BP (hostmaterial), and [Ir(tBuppm)₂(acac)] (guest material), which werecontained in the light-emitting layer of the light-emitting element 1 orthe comparative light-emitting element 2 in Example 1. The compositionor mass ratio of these materials was made to be different among thesamples.

The mass ratio in the organic film was such that 2mDBTBPDBq-II: PCBBiNB(or PCBA1BP): [Ir(tBuppm)₂(acac)]=1−X:X:0.06. Table 3 shows thecomponents and the emission lifetimes of the samples.

TABLE 3 Emission Host τ₁ ^(a) τ₂ ^(b) lifetime ^(c) Sample material X[μsec] [μsec] [μsec] 1 PCBBiNB 0 1.15 — 5.3 2 PCBBiNB 0.2 1.01 1.81 6.43 PCBBiNB 0.5 0.96 2.21 7.7 4 PCBBiNB 1 1.00 3.42 11.1 5 PCBA1BP 0.51.20 — 5.4 ^(a) Lifetime of the first decay component. ^(b) Lifetime ofthe second decay component. ^(c) Time required to decay to 1/100 of theinitial value.

The measurement was performed using a streak camera (C4334 manufacturedby Hamamatsu Photonics K.K.) with an N₂ gas laser (MSG 800 manufacturedby Laser Technik Berlin, λ=337 nm, pulse width <500 ps, repetitionrate=10 Hz). The hole size was set to 100 μm, and the measurement timewas set in a range of 0 μsec to 20 μsec. FIG. 16 shows the measurementresults. Here, τ₁ and τ₂ each are the lifetimes of the decay componentswhich are divided according to Formula 1. From Table 3, it can beconcluded that Samples 2 to 4 give a multicomponent (two components)decay curve.

The analysis of the data shown in FIG. 16 indicates that, in the case ofSample 5 where PCBA1BP whose T1 level is higher than that of[Ir(tBuppm)₂(acac)] serving as a guest material is used as a hostmaterial (PCBA1BP: 50%), only a single decay component is observed. Onthe other hand, in the case of Sample 2 (PCBBiNB: 20%), Sample 3(PCBBiNB: 50%), and Sample 4 (PCBBiNB: 100%) where PCBBiNB whose T1level is lower than that of [Ir(tBuppm)₂(acac)] serving as a guestmaterial is used as a host material, two decay components (a first decaycomponent and a second decay component) are observed and theirlifetimes, which are the time required for the initial emissionintensity to decay to 1/100 of the initial values, are less than orequal to 15 μsec. Furthermore, it is found that the emission lifetime(τ1) of the first decay component in each of Samples 2 to 4 is shorterthan that in Sample 5; the emission lifetime (τ2) of the second decaycomponent in Sample 4 having the highest proportion of PCBBiNB is thelongest among Samples 2 to 4. The second component is considered to becontributed by the emission process via the energy transfer from thehost material to the guest material.

Thus, the features of the light-emitting element of one embodiment ofthe present invention are that a light-emitting layer including a guestmaterial and a host material having a T1 level lower than that of theguest material is included and that the emission from the light-emittinglayer shows a multicomponent decay curve as shown in FIG. 16.

EXPLANATION OF REFERENCE

-   11: host material, 12: first guest material, 101: anode, 102:    cathode, 103: EL layer, 104: light-emitting layer, 105: first    organic compound (serving as a host material), 106: second organic    compound (serving as a first guest material), 107: third organic    compound (serving as a second guest material), 201: first electrode,    202: second electrode, 203: EL layer, 204: hole-injection layer,    205: hole-transport layer, 206: light-emitting layer, 207:    electron-transport layer, 208: electron-injection layer, 209: first    organic compound (serving as a host material), 210: second organic    compound (serving as a first guest material), 211: third organic    compound (serving as a second guest material), 301: first electrode,    302(1): first EL layer, 302(2): second EL layer, 304: second    electrode, 305: charge-generation layer, 305(1): first    charge-generation layer, 305(2): second charge-generation layer,    501: element substrate, 502: pixel portion, 503: driver circuit    portion (source line driver circuit), 504 a, 504 b: driver circuit    portion (gate line driver circuit), 505: sealant, 506: sealing    substrate, 507: wiring, 508: FPC (flexible printed circuit), 509:    n-channel FET, 510: p-channel FET, 511: switching FET, 512: current    control FET, 513: first electrode (anode), 514: insulator, 515: EL    layer, 516: second electrode (cathode), 517: light-emitting element,    518: element layer, 1100: substrate, 1101: first electrode, 1102: EL    layer, 1103: second electrode, 1111: hole-injection layer, 1112:    hole-transport layer, 1113: light-emitting layer, 1113 a: first    light-emitting layer, 1113 b: second light-emitting layer, 1114:    electron-transport layer, 1115: space, 7100: television device,    7101: housing, 7103: display portion, 7105: stand, 7107: display    portion, 7109: operation key, 7110: remote controller, 7201: main    body, 7202: housing, 7203: display portion, 7204: keyboard, 7205:    external connection port, 7206: pointing device, 7301: housing,    7302: housing, 7303: joint portion, 7304: display portion, 7305:    display portion, 7306: speaker portion, 7307: recording medium    insertion portion, 7308: LED lamp, 7309: operation key, 7310:    connection terminal, 7311: sensor, 7312: microphone, 7400: mobile    phone device, 7401: housing, 7402: display portion, 7403: operation    button, 7404: external connection port, 7405: speaker, 7406:    microphone, 8001: lighting device, 8002: lighting device, 8003:    lighting device, 8004: lighting device, 9033: clasp, 9034: display    mode switch, 9035: power supply switch, 9036: power saver switch,    9038: operation switch, 9630: housing, 9631: display portion, 9631    a: display portion, 9631 b: display portion, 9632 a: touch panel    region, 9632 b: touch panel region, 9633: solar cell, 9634:    charge/discharge control circuit, 9635: battery, 9636: DC-DC    converter, 9637: operation key, 9638: converter, 9639: button

This application is based on Japanese Patent Application serial no.2013-063634 filed with Japan Patent Office on Mar. 26, 2013, the entirecontents of which are hereby incorporated by reference.

1. A light-emitting device comprising: a first electrode; alight-emitting layer over the first electrode, the light-emitting layercomprising a host material, a first guest material, and a second guestmaterial; and a second electrode over the light-emitting layer, whereina T1 level of the host material is lower than a T1 level of the firstguest material and higher than a T1 level of the second guest material,and wherein a difference in T1 level between the host material and thefirst guest material is larger than 0 eV and less than or equal to 0.2eV.
 2. The light-emitting device according to claim 1, wherein the firstguest material and the second guest material each are an organometalliccompound.
 3. The light-emitting device according to claim 1, wherein arate constant of a radiative transition of the second guest material islarger than 5×10⁵ sec⁻¹.
 4. The light-emitting device according to claim1, wherein a rate constant of a non-radiative transition of the hostmaterial is smaller than 1×10² sec⁻¹.
 5. The light-emitting deviceaccording to claim 1, wherein an emission from the light-emitting layerexhibits a multicomponent decay curve, and wherein a lifetime of theemission is less than or equal to 15 μsec where the lifetime is a timerequired for the emission to decrease in intensity to 1/100 of aninitial value thereof.
 6. The light-emitting device according to claim5, wherein the lifetime of the emission is less than or equal to 10μsec.
 7. The light-emitting device according to claim 5, wherein thelifetime of the emission is less than or equal to 5 μsec.
 8. Anelectronic device comprising the light-emitting device according toclaim
 1. 9. A lighting device comprising the light-emitting deviceaccording to claim
 1. 10. A light-emitting device comprising: a firstelectrode; a first EL layer over the first electrode; a chargegeneration layer over the first EL layer; a second EL layer over thecharge generation layer; and a second electrode over the second ELlayer, wherein at least one of the first EL layer and the second ELlayer comprises a light-emitting layer which comprises a host material,a first guest material, and a second guest material, wherein a T1 levelof the host material is lower than a T1 level of the first guestmaterial and higher than a T1 level of the second guest material, andwherein a difference in T1 level between the host material and the firstguest material is larger than 0 eV and less than or equal to 0.2 eV. 11.The light-emitting device according to claim 10, wherein the first guestmaterial and the second guest material each are an organometalliccompound.
 12. The light-emitting device according to claim 10, wherein arate constant of a radiative transition of the second guest material islarger than 5×10⁵ sec⁻¹.
 13. The light-emitting device according toclaim 10, wherein a rate constant of a non-radiative transition of thehost material is smaller than 1×10² sec⁻¹.
 14. The light-emitting deviceaccording to claim 10, wherein an emission from the light-emitting layerexhibits a multicomponent decay curve, and wherein a lifetime of theemission is less than or equal to 15 μsec where the lifetime is a timerequired for the emission to decrease in intensity to 1/100 of aninitial value thereof.
 15. The light-emitting device according to claim14, wherein the lifetime of the emission is less than or equal to 10μsec.
 16. The light-emitting device according to claim 14, wherein thelifetime of the emission is less than or equal to 5 μsec.
 17. Anelectronic device comprising the light-emitting device according toclaim
 10. 18. A lighting device comprising the light-emitting deviceaccording to claim 10.